It is known to treat particles with a fluid by passing the particles as a body in one direction and passing the fluid with which they are to be treated countercurrent to the particles. In the simplest method the particles are advanced along a support that is foraminous, that is either perforated or otherwise gas-pervious. As the particles more along this foraminous support the fluid for treating them is passed upwardly through the support and through the body of particles. This action fluidizes the body of particles, as the upwardly moving fluid at least partially counteracts the downwardly effective force of gravity so as to suspend these particles and convert the body of particles into a highly fluent mass. Such a system ensures intimate contacting of the fluid and the particles for excellent treatment of these particles, usually for drying of wet particles.
With particles of small surface density, that is ratio of surface area to mass, it is necessary to work with a relatively low fluid speed. If the fluid speed exceeds a predetermined limit for a given particle size the fluid will simply entrain the particle. For this reason a vigorous fluid flow can only be used with particles of relatively great surface density, so that the overall efficiency of such a system falls off directly with particle size. Thus the installations which serve to dry particles of very small surface density are inherently quite large and inefficient in operations.